In recent years, a high efficiency coding is performed in order to compress and transmit an audio (voice) signal efficiently. An algorithm for such audio compression is standardized, for example, by MPEG (Moving Picture Expert Group).
The audio compression algorithm of MPEG is known as MPEG2 AAC (MPEG2 Advanced Audio Codec: “ISO/IEC 13818-7 Part7: Advanced Audio Coding (AAC)”) and MP3 (MPEG1 Audio Layer 3: “ISO/IEC 11172-3 Part3: Audio”) and so on.
MPEG 2 AAC has been broadly applied, for example, to ISDB standard for BS digital broadcasting and terrestrial digital broadcasting in Japan, AAC format for SD-Audio, and DVB (Digital Video Broadcasting) in the Europe bloc, and so on.
In quantization process of coding algorithm of AAC, repetitive loop processes which are referred to as an inner loop and an outer loop are performed to satisfy given bit rate (the number of usable bits for quantization).
In inner loop, scalefactor is controlled to adjust quantization granularity so that quantization error is masked based on human's auditory property. In outer loop, in order to control overall code amount, common scale (common scale value) is controlled to adjust quantization granularity of overall frame.
Such two kinds of values for determining quantization granularity (scalefactor and common scale) may have a large influence on coding quality, and therefore inner loop and outer loop are demanded to be controlled at the same time efficiently, correctly.
For example, written standards (ISO/IEC 13818-7) of MPEG-2 AAC introduce a manner of controlling scalefactor and common scale arbitrarily at the time of quantization. Outer loop (bit control loop) for controlling common scale changes, for example, common scale by one quantization step at a time, and repeats loop until the number of quantization bits is equal to or less than the number of usable bits for quantization.
However, when bit control loop is repeated while common scale is changed by one step at a time, it is difficult to complete quantization process within a short time. Addressing such problem, there was an attempt at estimating common scale by which the number of quantization bits becomes equal to or falls under target value, based on actual result value of quantization bit. However, it is difficult to obtain target common scale within a short time due to various factors.
In the related art, there have been proposed various kinds of audio signal coding devices and audio signal coding methods.    Patent Document 1: Japanese Laid-open Patent Publication No. 2008-065162    Non-Patent Document 1: INTERNATIONAL STANDARD, “ISO/IEC 13818-7 Part 7: Advanced Audio Coding (AAC),” Fourth edition, 2006-01-15    Non-Patent Document 2: INTERNATIONAL STANDARD, “ISO/IEC 11172-3 Part 3: Audio,” First edition, 1993-08-01